The present invention relates to a method of adjusting the resonant frequency of a piezoelectric resonator.
Certain crystals, such as quartz crystals, have the ability to generate a voltage when a mechanical force is applied thereto. Conversely, such crystals also have the ability to deform when a voltage is applied to the crystal. These crystals are commonly referred to as piezoelectric crystals.
Piezoelectric crystals have highly stable frequency characteristics, and, therefore, they are frequently used as resonators in oscillator circuits.
Quartz crystal micro-resonators can be advantageously employed as the time standard of a timepiece. The small size permits the micro-resonator to be housed together with a micro-electronic oscillator, frequency divider and driver circuitry in a package sufficiently small to fit in a wristwatch.
Although quartz crystal micro-resonators can be batch fabricated to frequencies relatively close to a desired value, it is necessary to tune or trim each crystal individually to obtain an exact resonant frequency.
For example, quartz crystal resonators can be cut by known manufacturing procedures to an accuracy of plus or minus 1 Hz which is satisfactory for many purposes. However, in applications such as quartz watches, it is essential that the resonant frequency of the quartz resonator be a predetermined frequency to permit accurate timekeeping.
Prior art processes for tuning crystal resonators are generally too slow, expensive and inaccurate.
For example, one prior art process includes manually rubbing a crystal plate against a sheet of abrasive paper, cleaning the crystal plate and then measuring the resonant frequency of the resonator. This process is most unacceptable since it involves a trial and error process which requires skilled labor.
Another known method involves using a stream of abrasive particles or powder to reduce the length of the crystal plate. The length reduction operation is automatically controlled. However, this technique is little more than automating the above mentioned trial and error process of using an abrasive to reduce the size of the crystal plate. This process is undesirable since it permits unidirectional tuning only, i.e., increasing the resonant frequency by reducing crystal size and does not enable or permit the lowering of the resonant frequency, and because the tuning is effected prior to mounting of the crystal in its can receptacle.
A still further prior art method of tuning a crystal resonator involves mounting the resonator in a package having a transparent window region through which the crystal micro-resonator is tuned by use of a laser beam to remove portions of (metal) weights formed or provided on the tine ends of a tuning fork type crystal resonator. The process is undesirable since it too enables unidirectional tuning only and because it requires the added and expensive process of providing weights on tuning fork tine ends and a transparent window region in the crystal container.
The prior art includes patents of Beaver U.S. Pat. No. 3,808,752, Staudte U.S. Pat. No. 3,766,616, Keeler U.S. Pat. No. 3,803,828, Zemla U.S. Pat. No. 3,046,460, Wiley U.S. Pat. No. 3,306,030, Nakai U.S. Pat. No. 3,469,389, Hansell U.S. Pat. No. 1,874,982, Assmus U.S. Pat. No. 3,805,509, Hund U.S. Pat. No. 1,886,814 and Oguchi U.S. Pat. No. 3,906,260. These patents are mentioned as being representative of the prior art and other pertinent patents may exist. None of the above cited patents are deemed to effect the patentability of the claimed invention.